The invention relates to a filter system, especially for the lubricant of an internal combustion engine, having at least two filter elements.
U.S. Pat. No. 5,298,158 discloses a dual filter system in which two filter elements are provided, one filter element being equipped as a full-flow filter and the second filter element as partial flow filter. The partial flow filter element is provided with a valve which reduces the partial flow to a set value. Thus different amounts of the fluid to be filtered flow through the two filters, but basically in a parallel manner. Especially when such filter systems are applied to internal combustion engines, the lubricant produces very great thermal stress resulting in the fact that the filter elements must be replaced after a certain period of time. This requires greater maintenance service in addition to the cost of the replacement elements.
Published German Patent Application No. DE 1,178,829 discloses a changeover system for a dual filter for turning on and off each individual filter element. The changeover at the same time produces a flushing with recycled flushing filtrate and the discharge of the dirt. The disadvantage of such systems is that operating errors can occur which cause unclean filtrate to get into the clean fluid area. Thus, in the state of the art, a changeover apparatus is proposed wherein the changeover apparatus consisting of two rotary cylinders is provided with control slots. This is intended to prevent changeover errors. One disadvantage of the filter system described in the state of the art consists, however, in the fact that the manual changeover also requires more work in maintenance. Moreover, the cost of the components for the prevention of changeover errors is considerable.